Reinforced elastomeric articles are well known in the art. For example, conveyor or like type belts, tires, etc., are constructed with cords of textile and/or fine steel wire filaments or strands. In particular, belts used in pneumatic tires are constructed of up to eight ply layers with the cord reinforcement of adjacent plies being biased with respect to the direction of movement of the tire where it is desired to reinforce in both the lateral direction and the direction of rotation of the tire. Further, cords made of strands of multi-twisted filaments of fine wire with a single strand construction having two or more filaments and a wrap filament thereabout to reinforce the cord structure have also been known. In some cases, the reinforcement includes the use of single strand cords of multi-filaments which are not twisted about each other but rather twisted altogether as a bundle or bunch (bunched cord) to simplify the cord construction, as disclosed in assignees's U.S. Pat. No. 4,947,636 which is incorporated by reference in its entirety herein. Higher fatigue life requirements for composites in tires have resulted in cords with smaller filament diameter requiring more filaments in the cord to obtain the necessary strength.
Two ply tire belts for passenger and light truck tires can have cords of 2×0.255ST and 2+2×0.32-0.40ST, respectively. An example of the first construction is described in Assignee's Statutory Invention Registration H1333, issued Jul. 5, 1994, which application is incorporated by reference in its entirety herein, wherein multi-filament cords such as 2×0.255ST are disclosed. This designation means one cord of two (2) 0.255 mm. diameter filaments. An example of the 2+2×0.32-0.40ST cord is disclosed in Assignee's U.S. Pat. No. 5,242,001, which is incorporated in its entirety by reference herein. This designation means one cord of four (4) 0.32-0.40 mm. diameter filaments (with two (2) filaments twisted at a shorter lay length than the other two (2) filaments). Multi-filament cords such as 2+2×0.32-0.40ST have been found necessary to meet the higher demand of strength for composites in tire belts, typically used in light truck applications. Both of these cords were made of super tensile (ST) steel as defined hereinafter. Though cord designs incorporating super tensile (ST) steel have proven effective, there is a continuing need to develop lighter weight cord constructions with improved characteristics, such as higher corrosion propagation resistance and improved tire performance, over recent high tensile and super tensile constructions.
The described cord constructions generally have not found use in larger tires, such as off-the-road (OTR) tires, because they were not strong enough. Even with the advent of high tensile filament such as in Assignee's 2+2× cord, disclosed for use in passenger and light truck tires, the large OTR tires continue to use traditional constructions such as 7×7×0.25+1HT and 3×7×0.22HE comprising seven strands each of seven 0.25 mm diameter high tensile filaments that are twisted together and spiral-wrapped; and three strands each of seven 0.22 mm diameter high tensile filaments that are twisted together, respectively. The steel cord cable currently used for ply reinforcement in OTR tires for sizes 36.00R51 and larger is stranded cord of high tensile tire cord filament such as 7×19×0.20+1HT cord comprising seven strands each of nineteen 0.20 mm diameter high tensile filaments that are twisted together and spiral-wrapped. These cords were made of high tensile (HT) steel as defined hereinafter.
More recently, OTR tires can be constructed of multiple plies belts or single ply with reinforcing cords such as 27×0.265ST or 5+8+14×0.265ST+1 as disclosed in Assignee's U.S. Pat. No. 5,318,643 which patent is incorporated by reference in its entirety herein. Still, current steel cord constructions have breaking load and cable gauge limitations preventing the needed design inch-strength from being achieved for tires larger than 40.00R57 used on trucks and earthmovers weighing up to and sometimes more than 320 tons. In addition, there is a need to increase the rivet area in the ply and belt, i.e., the space between the cords, for tire sizes of 36.00R51 and larger so that more rubber can penetrate between the cords during tire manufacture to enhance the quality of calendered treatment by preventing “weak rivet” or “loose coat” (which can result in trapped air in tires).
Many problems have had to be overcome even after development of the above higher strength filaments and cords. The higher strength steel alloys resulted in changes in cord modulus giving rise to the possibility of adjusting the parameters of a tire belt gross load which depends upon three factors assuming adequate cord to rubber adhesion. The factors are cord modulus, the ratio of cord volume to rubber volume (often expressed as the number of cord ends per inch (epi)), and the angle of cord reinforcement. Further, as the angle of cord reinforcement approaches the direction of rotation of the tire, the support from the reinforcement in the lateral direction moves toward zero. An increase in the above-mentioned two other cord related factors, i.e., the cord modulus and the ratio of cord volume to rubber volume, generally results in an increase of weight for the belt. Added weight can mean added cost, higher rolling resistance and lower fuel economy of a tire. Simply using lighter cords with a lower modulus does not solve the problem because, even though they have lower weight, the lower cord modulus must be offset by increasing the ratio of cord to rubber volume. This increase in cord volume is limited by the physical size of the cord and the resulting spacing between the cords which governs the amount of rivet, i.e., the ability of the rubber to penetrate between the cords for good cord to rubber adhesion.